Flow past a rotating circular cylinder is studied experimentally. The experiments are carried out in a water tunnel at Reynolds numbers of 200, 300, and 400 and nondimensional rotation rates (ratio of surface speed of the cylinder to the free stream velocity), α, varying from 0 to 5. The diagnostic is done by flow visualization using hydrogen bubble technique and quantitative measurements using a particle image velocimetry technique. We present the global view of the wake structure at the three Reynolds numbers and various rotation rates. Vortex shedding activity is observed to occur from α=0 to α~1.95, after which it is suppressed. Reynolds number is found to have a strong effect on the wake morphology near the suppression rotation rate, α=1.95. Interestingly, the vortex shedding activity again resumes in the range 4.34<α<4.70 as first discovered numerically (Mittal and Kumar, 2003, “Flow past a rotating cylinder,” J. Fluid Mech., 476, 303) for Re = 200. The shed vortices are of one sign in this range of rotation rates. Experimental evidence of this new vortex shedding mode is presented, for the first time, at α=4.45 in the newly discovered window of rotation rates, using flow visualization and particle image velocimetry measurements. Strouhal number measurements and global wake patterns agree well with the computations of Mittal and Kumar at a Reynolds number of 200.
Flow around two circular cylinders in a side-by-side arrangement normal to the free stream with heat release from one of the cylinders is studied experimentally. This flow, with no heat release, is known to exhibit a range of flow regimes at different cylinder spacings. In particular, the wake exhibits well-known intermittently bistable behavior in the center-to-center spacing (normalized by cylinder diameter) range of 1.2-2.0. We present, for the first time, the effect of heat release from one of the cylinders on the near-wake structure of the two cylinder configuration. The experiments are performed at spacing ratios of 1.1, 1.7, and 3.0, Reynolds numbers of 250, 350, and 450 and Richardson number less than 0.14. The investigations are carried out in a water tunnel using hydrogen bubble technique for flow visualization and particle-image-velocimetry for quantitative measurements. The bistability of the wake at a spacing ratio of 1.7 is controlled with a threshold heat release from one of the cylinders resulting in a stable narrow wake behind the heated cylinder and a wider wake behind the unheated cylinder. The heat release resulted in deflection of the gap-bleeding flow toward the heated cylinder at spacing ratio of S/D=1.1 and did not produce any visual changes in the near-wake structure at spacing ratio of 3.0.
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